1. Field of the Invention
The present invention relates to electrochemical hydrometallurgy for reducing noble metal waste, and more particularly to a method and apparatus for extracting noble metals from inorganic granular waste catalysts.
2. Description of Related Art
A method for extracting noble metals from inorganic granular waste catalysts means a method comprising: electrochemically leaching noble metals in an electrolytic cell; precipitating the noble metals in a cathode; and then separating the noble metals from the cathode.
In a prior method for dissolving and extracting noble metals from waste catalysts [Prior-Art Document 1: U.S. Pat. No. 4,775,452, 1988, “Process for dissolution and recovery of noble metals”], leaching is carried out in the anode chamber of a horizontal type electrolytic cell. The horizontal type electrolytic cell comprises a fluorine resin-based anion exchange membrane that separates the electrolytic cell into two chambers, anode and cathode chambers. The bottom of the anode chamber comprises a diffusion lattice. In the first step of extracting noble metals, a granular waste catalyst fixed bed is introduced into the anode chamber, and an electrolyte is circulated upward through the diffusion lattice. As the electrolyte, hydrochloric acid, nitric acid, sulfuric acid or an acidic compound is used. Preferably, a 5-35% hydrochloric acid is used. Herein, the anode and cathode membranes are positioned along the side of the electrolytic cell in parallel with the flow direction of the electrolyte.
The porous anode of stable size is made of titanium coated with noble metal oxide. The cathode is made of titanium. The electrolytic cell is 85 mm in length, 115-250 mm in width and 200-1000 mm in depth. In the second step after leaching noble metals, the electrolyte is 6-50-fold diluted and the noble metals are precipitated, whereby the noble metals are separated to activated carbon granules present in a fluidized state in the cathode space of a second electrolytic cell including a cationic membrane.
The disadvantage of this extraction method is that the efficiency for extracting noble metals decreases as the distance between the anode and the cathode increases. This is because hydrochloric oxide moves upward in parallel with the anode membrane by the electrolyte flow and its concentration decreases as it goes away from the surface of the anode membrane toward the cathode. For this reason, the leaching of novel noble metals is mainly performed in the anode bed close to waste catalysts.
Because the electrolyte is pumped once through the electrolytic cell, a large amount of solution flows out, such that additional equipment is required, thus increasing economic losses.
The apparatus that is used to realize the extraction method according to Prior-Art Document 1 is energy-intensive, has low efficiency in extracting noble metals and requires the use of high-concentration (5-35%) acid (mainly hydrochloric acid).
A prior art for extracting noble metals from inorganic waste catalysts, sludge, ore concentrates and other metals [Prior-Art Document 2: Russia Patent No. 21199646, 1997, “Method for extracting noble metals and apparatus for carrying out the same”] has a characteristic in that the leaching of noble metals and the precipitation of a filled cathode during the circulation of an electrolyte through a fixed filter bed or fluidized bed of leached particles are carried out simultaneously in the same step.
The extraction of noble metals is carried out simultaneously through an electrolyte cell including a leaching block and a filled cathode. A 10-25% sodium chloride aqueous solution containing a required amount of hydrochloric acid and alkali is used as an electrolyte. Herein, noble metals are deposited on the filled cathode. The leaching block comprises one or several reactors which are provided with conventional units for introducing and discharging a leaching material. The leaching block includes an electrolyte cell provided with a pH-measuring chamber and an automatic discharge control unit.
After noble metals have been deposited, the filled cathode is separated from the electrolytic cell and sent to a recycling process. For metal extraction, the filled cathode is incinerated. Metal extraction may also be performed without separating the cathode from the electrolytic cell. In this case, noble metals are dissolved by passing an electric current of opposite polarity through the cathode, thus obtaining a high-concentration chloride solution.
The disadvantages of the method according to Prior-Art Document 2 are that the leaching process is complicated and the functional technical blocks are separated from each other to make the design of the apparatus difficult.
The prior art for extracting noble metals from inorganic waste catalysts, ore concentrates and other metals [Prior-Art Document 3: Russia Patent No. 21989477, Sep. 12, 2000, “Method for extracting noble metals”] is technically closest to the present invention and comprises carrying out leaching in an electrolyte, circulating the electrolyte along a closed circuit through a filling material, precipitating metals in an electrolytic cell, and then separating noble metals from a cathode according to a conventional method, wherein the metals treated in the filled form are placed in the space between the electrodes of the electrolytic cell. The electrochemical leaching of noble metals can be activated by previously developing the polarity reversal of the electrodes. For this purpose, the electrodes are changed into a large-capacity multipolar electrode which allows the anode dissolution of metals regardless of the amount of material. Meanwhile, by inhibiting the formation of a brown cloud in the cathode, hydrated anionic chloride compounds of noble metals which are formed in a process of leaching the filling material are prevented from being burned out destroyed by a fire with the cathode, and the electrolyte is circulated from the anode to the cathode at a rate suitable for such conditions. Herein, acidic water containing 0.3-4.0% hydrochloric acid is used as the electrolyte.
In order to study the efficiency of said noble metal extraction method and examine the disadvantages thereof, the present inventors constructed an electrolytic cell (FIG. 1) corresponding to the description of Prior-Art Document [3]. As described in Prior-Art Document [3], the electrolytic cell has a horizontal structure, the effective cross-sectional area of the electrolytic cell is 1600 cm2 (40 cm×40 cm), and the length of the filling material is 100 cm. The filling material in the space between the electrodes is fixed with a dielectric lattice. The parameters of the experiment are consistent with those described in Prior-Art Document [3].
According to a study conducted by the present inventors using said prototype, the influence of polarity reversal on the rate and depth of leaching was insignificant. The leaching time increased by the time during which the polarity was developed. Also, noble metals were not formed as a compact foil on the surface of the titanium cathode and were precipitated in the form of niello which was easily separated from the cathode surface by rising hydrogen bubbles. Hydrogen bubbles separated from the surface of the cathode membrane rose to the surface of the electrolyte and formed convection current. As a result, the noble metal niello in a fluidized-bed state was placed in the cathode space of the electrolytic cell. Such conditions make the noble metal niello returning to the filling material of the waste catalyst through the lattice holes. In addition, the noble metal niello moves to the anode space of the electrolytic cell by the electrolyte flow being circulated. The filling material sample was analyzed after conducting the experiment, and as a result, it could be seen that the leaching of noble metals at the bottom of the filler was incomplete. This is because the rate of circulation of the electrolyte from the anode to the cathode is not constant along the cross section of the electrolytic cell. The electrolyte circulation rate is slower in the lower portion of the electrolytic cell than in the upper portion. This can be clearly explained because the waste catalyst particles of the lower portion of the electrolytic cell are under the pressure of the particles of the upper portion. This reduces the size of the free space in which the electrolyte circulation between the particles of the lower portion of the electrolytic cell occurs. Such conditions impose limitations on increasing the depth of the electrolytic cell in order to use the electrolytic cell in industrial applications. In addition, the area of the electrolytic cell in which the electrolyte evaporates is large. If the above-described process is carried out at 70° C., anode hydrochloric acid oxide actively evaporates, and thus additional means for reducing negative effects on the environment are required. Also, the acidity of the solution decreases, because hydrochloric acid is used to partially dissolve catalysts in the electrolytic process. It was found that, when the acidity (pH) of the solution was more than 1, the rate of leaching significantly decreased.
In order to maintain acidity at a constant level, it is required to periodically discharge the electrolyte from the electrolytic cell and to supplement hydrochloric acid to a required concentration.